Method for treatment of a gaseous mixture comprising hydrogen and hydrogen sulphide

ABSTRACT

A method of treatment of a gas mixture which contains H 2 , H 2 S and possibly of other impurities, such as hydrocarbons. The treatment method is aimed at purifying the hydrogen mixture without incurring a overall pressure loss. The method according to the invention uses a device for pressure swing adsorption where the device includes an integrated compressor. In each adsorber of the device a pressure swing cycle is carried out which includes the steps of: adsorption, decompression, purge, and repressurization.

BACKGROUND

The present invention relates to a process for treating a gas mixturecomprising hydrogen, H₂S and other components, such as hydrocarbons,with a view to separating the hydrogen from the other components of themixture while maintaining it at the pressure of the initial gas mixture.

The present invention relates to the treatment of products coming fromhydrotreatment (HT) processes very widely employed in the refiningindustry. Various types of hydrotreatment (HT) processes coexist in mostrefineries and are used for treating a large number of refiningproducts, particularly the following cuts: gasoline, kerosene, diesel,vacuum distillates, oil bases. These hydrotreatment processes are usedto adjust certain properties of the refining products, such as thecontents of sulfur, nitrogen and aromatic compounds, or the cetanenumber. Sulfur is often the key property (the units are also oftenreferred to as HDS (standing for hydrodesulfurization) units) and is thesubject of increasingly stringent specifications leading refiners toseek ways of improving these units.

Chemical hydrogenation reactions take place in a reactor in which thehydrocarbon feed is mixed with a stream of hydrogen (in large excess)and passes over a catalyst bed. Some of the hydrogen reacts with theunsaturated organosulfur and organonitrogen compounds, producinghydrogen sulfide (H₂S), ammonia (NH₃), C₁-C₆ light hydrocarbons (HC) andsaturated heavier compounds. On the downstream side of the reactor, aliquid/vapor separation tank is used to recover the hydrogen-rich gasphase, which is recycled in order to create this hydrogen excess(hereafter called the recycling gas). This gas contains, in addition tohydrogen, most of the volatile compounds that are created in the reactorand have a tendency to concentrate therein. The chemically consumedhydrogen, and also the hydrogen lost by mechanical losses, dissolutionor purging is compensated for by a hydrogen-rich make-up gas, thecomposition of which varies depending on its mode of production.Typically, the hydrogen content of this gas (hereafter called themake-up gas) varies between 70 mol % and 99.9 mol %, the remaindergenerally being methane or a mixture of light hydrocarbons.

An essential parameter of the hydrotreatment reaction is the hydrogenpartial pressure at the outlet of the reactor. This partial pressuredepends on the total pressure of the unit (set during the “design” ofthe unit), on the degree of vaporization of the hydrocarbon feed (set bythe total pressure and the operating temperature) and above all on thehydrogen concentration of the two gases—the make-up gas and therecycling gas—that are used. The H₂S partial pressure is a secondimportant parameter that depends mainly on the H₂S content of therecycling gas, and therefore on the sulfur of the feed, and on thedegree of desulfurization applied during the hydrotreatment. It istherefore desirable in hydrodesulfurization units to increase thehydrogen partial pressure and to reduce the H₂S partial pressure bypurifying either the make-up gas or the recycling gas, or both gases.The objective is therefore to reduce as far as possible the H₂S andhydrocarbon contents.

The prior art has already proposed various solutions for achieving thisobjective. Thus, a first solution consists in purging the recycling gasin order to limit its H₂S and hydrocarbon concentration: a fraction ofthe recycling gas is drawn off in order to remove the noncondensablegases that have built up in the recycling loop. These gases aredischarged into what is called the “fuel gas” line; this line is presentin all refineries and collects all the gaseous effluents that can beutilized in the form of energy. However, this high-pressure purge hasseveral drawbacks:

-   -   the impact on the hydrogen and H₂S partial pressures is slight;    -   since the recycling gas is rich in hydrogen, the primary        consequence of the purge is the loss of hydrogen to the “fuel        gas” line of the refinery. This hydrogen is then utilized since        it is employed as fuel; and    -   because of this loss, a larger amount of make-up gas has to be        introduced. However, the make-up gas is compressed in order to        go from the pressure of the hydrogen line to the operating        pressure of the unit. The high-pressure purge is therefore        limited by the capacity of the make-up gas compressor.

A second solution consists in employing a scrubbing step in which therecycling gas is scrubbed by an amine solution. During this scrubbing,H₂S is completely absorbed and then desorbed by regeneration of theamine, and finally converted into liquid sulfur, for example in a Clausunit placed downstream. However, the scrubbing relates only to H₂S andremoves none of the hydrocarbons from the recycling gas. The impact onthe H₂S partial pressure is substantial, but the impact on the hydrogenpartial pressure is negligible. The gain in hydrodesulfurizationperformance achieved thanks to this scrubbing therefore remains modest.In addition, the amine solutions pose corrosion and foaming problems.

A third solution of the prior art, which is very widely employed forH₂/CO/CH₄ mixtures, consists in purifying the hydrogen of the recyclinggas by adsorption. This adsorption is used to achieve purity levels ofhigher than 99.5%. The application of adsorption to ahydrodesulfurization recycling gas is, for example, disclosed in JP57055992. The adsorption treatment of the recycling gas or of therecycling gas mixture or of the make-up gas has substantial influence onthe hydrodesulfurization performance. However, this solution has neverbeen applied on an industrial scale to the treatment of these gasesbecause of low yields. This is because the hydrogen yield of adsorptionunits is generally between 70 and 90%. The loss of hydrogen musttherefore be compensated for by the use of a larger amount of make-upgas. The increase in the volume of make-up gas may be up to 100% in thecase of the treatment of the entire recycling gas by a PSA (PressureSwing Adsorption)-type process. The use of a PSA process thereforeinvolves a high hydrogen cost; this is also greatly limited by thecapacity of the make-up gas compressor and is in practice impossiblewithout extensive investment.

A fourth solution of the prior art is the recovery of the hydrogencontained in the recycling gas by treating this gas using ahydrogen-permeable membrane. This type of membrane makes it possible toobtain high hydrogen purities (90 to 98%) and acceptable yields (80 to98%, depending on the desired purity). The cost is moderate comparedwith the previous solutions. This solution has been described in thefield of hydrodesulfurization units in EP-A-061 259. This solution isapplied on an industrial scale, but the problem that remains is the needto recompress the recycling gas after it has passed through themembrane. This is because the purified hydrogen is produced at reducedpressure and the performance of the membrane is better the lower theproduction pressure. In practice, it is impossible to treat all therecycling gas. The membrane is therefore generally placed in a recyclinggas branch and the hydrogen produced is sent to the intake of themake-up gas compressor in order to return to the pressure of the unit.The volume treated by the membrane, and consequently its effectiveness,is once again limited by the capacity of the make-up gas compressor.

A fifth solution is the use of reverse-selectivity membranes, whichmaintain the hydrogen under pressure. However, these membranes have lowhydrogen/hydrocarbon selectivities (particularly in the case ofhydrogen/methane separation). They therefore allow all of the recyclinggas to be treated (as described in patents U.S. Pat. No. 6,190,540 andU.S. Pat. No. 6,179,996) in order to carry out more selectivehydrocarbon purging, but a compromise between the loss of hydrogen andthe degree of purification of the hydrogen has to be found. If theobjective is the high purification of the recycling gas (with a hydrogenpurity of 90%, or even 95%), then the hydrogen losses are verysubstantial (30% or 50%, or even higher) and the limitations are thesame as those of a simple purge, (corresponding to the abovementionedfirst solution). If the objective is to reduce the hydrogen losses incomparison with a conventional high-pressure purge (as in the abovefirst solution), then the purification and the impact on thehydrodesulfurization performance are very moderate. The last drawback isthat the loss of hydrogen varies with the composition of the recyclinggas to be treated—it is greater the richer the recycling gas to betreated is in hydrogen.

There is therefore a need to improve the hydrodesulfurization units ofthe prior art, and especially the treatment of the recycling gas and theuse of this gas and of the make-up gas. The object of the invention isto propose a process for treating the gas coming from ahydrodesulfurization unit so as to obtain a recycling gas having a highhydrogen purity without reducing the pressure of the gas and withoutloss of hydrogen during this treatment.

SUMMARY

The invention relates to a process for separating a gas mixture bypressure spring adsorption, in which, for the or each adsorber, apressure swing cycle comprising a succession of steps definingadsorption, decompression, purge and repressurization phases is carriedout.

The process for treating a gas mixture comprising at least H2S and H2and having a pressure P by means of a treatment device, comprises apressure swing adsorption (PSA) unit associated with an integratedcompressor, in which, for each adsorber of the PSA unit, a pressureswing cycle comprising a succession of phases that define adsorption,decompression, purge and repressurization phases, is carried out, inwhich:

-   -   during the adsorption phase, at the pressure P, the gas mixture        to be treated is brought into contact with the adsorber bed so        as to adsorb the different components of the hydrogen and to        produce, at the top of the adsorber bed, a gas comprising        essentially hydrogen;    -   during the decompression phase:        -   a gas compressed to the pressure P′ (<P), coming from the            compressor integrated into the treatment device, called the            recycle gas, is introduced into the adsorber bed and a PSA            waste gas is produced;    -   during the purge phase, a purge gas is produced;    -   and in which the recycle gas is either the compressed waste gas        or the compressed purge gas.

BRIEF DESCRIPTION OF THE DRAWINGS

For a further understanding of the nature and objects for the presentinvention, reference should be made to the following detaileddescription, taken in conjunction with the accompanying drawings, inwhich like elements are given the same or analogous reference numbersand wherein:

FIG. 1 illustrates one embodiment of the invention and its operatingcycle;

FIG. 2 illustrates a schematic of a conventional hydrodesulfurizationunit;

FIG. 3 illustrates one embodiment of the invention for treating ahigh-pressure gas mixture from a high-pressure separation unit of ahydrodesulfurization unit;

FIG. 4 illustrates a similar embodiment to that shown in FIG. 3, whereinthe mixture is at the intake of a recycling compressor;

FIG. 5 illustrates another embodiment of the invention wherein themethod is used for treating a compressed recycling gas delivered by therecycling compressor; and

FIG. 6 illustrates another embodiment of the invention wherein the gasmixture consists of the recycling gas and a make-up gas from a hydrogenline.

DESCRIPTION OF PREFERRED EMBODIMENTS

The invention relates to a process for separating a gas mixture bypressure spring adsorption, in which, for the or each adsorber, apressure swing cycle comprising a succession of steps definingadsorption, decompression, purge and repressurization phases is carriedout. The invention can be employed with PSA cycles in which theadsorption is carried out at a pressure substantially above atmosphericpressure, typically around 3 to 50 bar, whereas the minimum pressure ofthe cycle is substantially equal to atmospheric pressure, or to apressure of a few bar. These processes comprise various combinations ofadsorption, decompression, purge and recompression steps in theadsorbers. In addition, in what follows, the terms “inlet” and “outlet”denote the inlet and outlet ends of an adsorber in adsorption phase; theterm “cocurrent” denotes the direction of flow of the gas through theadsorber during this adsorption phase, and the term “countercurrent”denotes the opposite direction of flow.

The invention specifically relates to a process for treating a gasmixture comprising at least H₂S and H₂ and having a pressure P by meansof a treatment device comprising a pressure swing adsorption (PSA) unitassociated with an integrated compressor, in which, for each adsorber ofthe PSA unit, a pressure swing cycle comprising a succession of phasesthat define adsorption, decompression, purge and repressurizationphases, is carried out, in which:

-   -   during the adsorption phase, at the pressure P, the gas mixture        to be treated is brought into contact with the adsorber bed so        as to adsorb the different components of the hydrogen and to        produce, at the top of the adsorber bed, a gas comprising        essentially hydrogen;    -   during the decompression phase:        -   a gas compressed to the pressure P′(<P), coming from the            compressor integrated into the treatment device, called the            recycle gas, is introduced into the adsorber bed and        -   a PSA waste gas is produced;    -   during the purge phase, a purge gas is produced;    -   and in which the recycle gas is either the compressed waste gas        or the compressed purge gas.

Other features and advantages of the invention will become apparent onreading the description which follows. In the rest of the description,the term “CPSA” will denote the treatment device comprising a pressurespring adsorption (PSA) unit associated with an integrated compressorused in the process according to the invention.

The object of the process according to the invention is to treat a gasmixture comprising at least H₂S and H₂, which may be a stream comingfrom a hydrodesulfurization process. This stream may also includehydrocarbons. In general, the gas mixture to be treated has atemperature between 20 and 80° C., preferably between 30 and 50° C. Ingeneral it has a pressure between 15 and 80 bar absolute, preferablybetween 20 and 50 bar absolute. The composition of this type of mixtureis usually the following:

-   -   at least 0.2 mol %, preferably at least 0.5 mol %, generally at        most 5 mol % and even more preferably between 1 and 2.5 mol %        H₂S,    -   between 60 and 98 mol % and preferably between 85 and 95 mol %        hydrogen;        and if hydrocarbons are present:    -   at most 20 mol % and preferably between 0.5 and 5 mol % CH₄;    -   at most 10 mol % and preferably between 0.1 and 3 mol % C₂        hydrocarbons;    -   at most 5 mol % and preferably between 0.05 and 1 mol % C₃        hydrocarbons;    -   at most 2 mol % and preferably between 0.02 and 0.5 mol % C₄        hydrocarbons; and    -   at most 0.5 mol % and preferably at most 0.06 mol % hydrocarbons        having a carbon number greater than or equal to 5.

According to the process for the invention, during the adsorption phase,the gas mixture to be treated is brought into contact with a firstadsorbent bed of the PSA unit: the gas mixture is introduced into thebottom part of the bed in the cocurrent direction. During thiscontacting step, the most adsorbable components other than H₂,essentially H₂S and possibly the hydrocarbons, if they are present inthe gas mixture to be treated, are adsorbed on the adsorbent and a gascomprising essentially hydrogen is produced at the pressure P reduced byabout 1 bar of pressure loss. During this step, the hydrogen producedgenerally has a purity of greater than at least 97 mol %, preferablygreater than at least 99 mol % or even greater than at least 99.5 mol %.In the case of the treatment of a gas coming from a hydrodesulfurizationunit, the gas obtained, comprising essentially hydrogen, may be recycledinto the hydrodesulfurization process since it possesses the hydrogenpurity and the pressure that are needed for this type ofhydrodesulfurization process.

To obtain effective purification, the adsorbent of the PSA beds must inparticular allow the adsorption and desorption of H₂S and of heavyhydrocarbons, such as pentane and hexane. The adsorbent bed is generallycomposed of a mixture of several adsorbents, said mixture comprising,for example, at least two adsorbents chosen from active carbons, silicagels, aluminas or molecular sieves. Preferably, the silica gels musthave a pore volume of between 0.4 and 0.8 cm³/g and a specific surfacearea of greater than 600 m²/g. Preferably, the aluminas have a porevolume of greater than 0.2 cm³/g and a specific surface area of greaterthan 220 m²/g. The zeolites preferably have a pore size of less than 4.2Å, with an Si/Al molar ratio of less than 5, and they contain Na and K.The active carbons preferably have a specific surface area of greaterthan 800 m²/g and a micropore size of between 8 and 20 Å.

According to a preferred embodiment, each PSA adsorbent bed is composedof at least three layers of adsorbents of different types. Each PSAadsorbent bed may comprise the following: in the bottom part, aprotective layer composed of alumina and/or of silica gel surmounted bya layer of active carbon and/or of carbon-containing molecular sieveand, optionally, in the top part, a layer of molecular sieve. Theproportions vary depending on the nature of the gas mixture to betreated (especially depending on its concentration of CH₄ and C₃₊hydrocarbons). For example, a gas mixture containing no water,comprising 75 mol % H₂, 5 mol % C₃₊ and 20 mol % light (C₁-C₂)hydrocarbons, CO and N₂, may be treated by an adsorption unit whose bedscomprise at least 10 vol % alumina and 15 vol % silica gel in the bottombed, the remainder being obtained by active carbon.

During the decompression phase of the process according to theinvention, several steps are employed, including at least a cocurrentfirst decompression step. After this cocurrent first decompression step,the decompression phase comprises a second step during which a gascompressed to the pressure P′ (<P), called the recycle gas, isintroduced cocurrently into the bed of the adsorber. This recycle gas iscompressed directly or indirectly by the compressor integrated into theCPSA. According to one essential feature of the invention, two streamsmay constitute this recycle gas, individually or as a mixture, namelythe waste gas coming from the PSA unit, which has been compressed, andthe purge gas coming from the PSA unit, which has been compressed.Preferably, this is the purge gas and not the waste gas. The waste gascomes from the final step of the PSA decompression phase and is partlycompressed by the compressor integrated into the PSA unit of the CPSAtreatment device, whereas the purge gas comes from the PSA purge phaseand is partly compressed by this same compressor integrated into the PSAunit, before being used as recycle gas. These two gases both comprisehydrogen and essentially H₂S, and possibly hydrocarbons if the gasmixture to be treated were to contain them, but in differentproportions. Introducing them into the adsorbent bed allows them to bereprocessed. Thus, when they come into contact at the pressure P′ withan adsorbent bed, on the one hand, the components other than hydrogenare adsorbed (thereby allowing the bed to be saturated with hydrocarbonsand with H₂S) and, on the other hand, a gas comprising essentiallyhydrogen, is produced as overhead of the adsorbent bed, said gastherefore having an H₂ purity close to that of the gas coming from theadsorption step described above.

The decompression phase of the process according to the invention alsogenerally includes at least one cocurrent decompression third step inwhich the adsorbent bed is brought to a pressure P″(<P′) (thereby makingit possible to reduce the hydrogen partial pressure of the internal gasphase of the adsorbent bed and to remove the hydrogen present in thedead volumes of the adsorber). This cocurrent decompression third stepof the decompression phase produces a gas stream comprising essentiallyhydrogen, and therefore with an H₂ purity close to that of the gascoming from the adsorption step described above, that is to say with ahydrogen purity at least greater than 97 mol %. These gas streamscomprising essentially hydrogen, that are produced during these secondand third steps of the decompression phase are generally used:

-   -   either during the purge phase;    -   or to recompress a downstream adsorbent bed in the        repressurization phase. The choice of the way in which they are        used is made according to their pressure and therefore their        possible use and not for recompressing a downstream adsorbent        bed. Preferably, the cocurrent decompression step of the        decompression phase, during which a gas stream comprising        essentially hydrogen is produced, corresponds to the step during        which a gas compressed to the pressure P′ (<P), coming from the        compressor integrated into the treatment device, called the        recycle gas, is introduced into the adsorber bed. The third step        of the decompression phase of the process according to the        invention also results in the production of the waste gas of the        PSA unit. According to the invention, this production of the        waste gas may be obtained by countercurrent decompression        initiated at the pressure P″ (<P′) of the PSA unit. This waste        gas comprises essentially H₂S, or even hydrocarbons, and is        depleted in hydrogen, that is to say it has a hydrogen content        of less than 25 mol %. This waste gas may be removed from the        process and burnt or reutilized as recycle gas in the process        according to the present invention as indicated above.

When the low pressure of the cycle has been reached, a purge phase iscarried out in order to complete the regeneration of the adsorber.During the purge phase, a gas is introduced countercurrently into theadsorber and a purge gas is produced. This purge gas generally comprisesbetween 40 and 85 mol % H₂. As indicated above, the gas introducedcountercurrently into the adsorber during the purge phase is a gasstream coming from one of the steps of the decompression phase. Afterrecompression, the purge gas is generally used as recycle gas; it may bemixed with a secondary gas source external to the CPSA unit, containingat least 30 vol % hydrogen before being compressed.

According to one particular process for implementing the invention, thepurge gas and/or the waste gas coming from the compressor may becompletely or partly treated in a hydrogen-permeable membrane beforebeing used as recycle gas. In general, a hydrogen-selective membrane isused, which produces a permeate rich in hydrogen and a retentate rich inH₂S and possibly hydrocarbons, depending on the nature of the gasmixture treated. It may be a membrane of the H₂S-resistant polymer type,for example one based on a polyimide or polyaramid, preferably apolyaramid (e.g. polyparaphehylene terephthalamide). The permeate gascoming from the hydrogen-permeable membrane may be mixed with one of thegas streams comprising essentially hydrogen coming from a cocurrentdepressurization step before its use in the purge phase.

During the repressurization phase, the pressure of the adsorber isincreased by countercurrently introducing a hydrogen-containing gasstream at a pressure greater than P″, such as the gas comprisingessentially hydrogen produced at the pressure P during the adsorptionphase and the gas comprising essentially hydrogen produced during thevarious steps of the decompression phase.

Preferably, the process involves at least four adsorbent beds placedcyclically under pressure one after the other. FIG. 1 illustrates oneoperating cycle of a CPSA unit comprising eight adsorbers (R01 to R08).In this figure, in which time t is plotted on the x-axis and absolutepressure P on the y-axis, the arrow-headed lines indicate the movementsand destinations of the gas streams and also the direction of flowthrough the adsorbers. When an arrow is in the direction of increasingpressure values (upward in the plot), the stream is said to flowcocurrently through the adsorber. If the upwardly pointing arrow liesbelow the line indicating the pressure in the adsorber, the streamenters the adsorber via the inlet end of this adsorber. If the upwardlypointing arrow lies above the line indicating the pressure, the streamleaves the adsorber via the outlet end of this adsorber, the inlet andoutlet ends being respectively those for the gas to be treated and forthe gas withdrawn in production/adsorption phase. When an arrow is inthe direction of decreasing pressure values (downward in the plot), thestream is said to flow countercurrently through the adsorber. If thedownwardly pointing arrow lies above the line indicating the pressure inthe adsorber, the gas stream enters the adsorber via the outlet end ofthe adsorber, the inlet and outlet ends again being those for the gas tobe treated and for the gas withdrawn in production/adsorption phase.Each adsorber R01 to R08 follows the cycle shown in FIG. 1, each beingshifted relative to the adsorber preceding it by a time called the“phase time” and equal to the duration T of the cycle divided by eight,that is to say divided by the number of adsorbers in operation. Thecycle shown in FIG. 1 therefore comprises eight phase times andillustrates the “phase time/adsorber” duality, that is to say that atany instant in the operation of the CPSA unit, each adsorber is in adifferent phase time.

Thus, the gas to be treated is introduced into the adsorber R08, whichis entering a first step of the adsorption phase at the pressure P,whereas the adsorber R07 is starting the second step of the adsorptionphase at the same pressure. During these two steps of the adsorptionphase, a gas comprising essentially hydrogen is produced.

The adsorber R06 is then in decompression phase. It undergoes acocurrent decompression first step down to the pressure P′: a gas streamis produced that is used for pressure balancing between the adsorbersR06 and R01. At the pressure P′, the recycle gas is introducedcocurrently into the adsorber R06. During this step, the componentsother than hydrogen, such as H₂S and hydrocarbons, contained in therecycle gas are adsorbed and a gas comprising essentially hydrogen isproduced; this is a recycling step. The hydrogen produced in R06 isemployed as gas for repressurizing the adsorber R02. The recycle gas isa gas compressed by the compressor integrated into the CPSA unit andcomes, upstream of the compressor, from the adsorber R03 in purge phase.Successive cocurrent decompression of the adsorbers R04 and R05 allowsthe pressure P′ to be lowered to the saturation pressure P″ of theadsorber R04. The decompression is carried out in two phases: firstly,by pressure balancing between the adsorbers R05 and R02 and secondly bysuccessive cocurrent decompression of the adsorbers R05 and R04 in orderto feed the downstream adsorber R03 in purge phase. The step ofproducing the waste gas of the CPSA unit, which is highly depleted inhydrogen, is then initiated by countercurrent depressurization of R04.The H₂S and hydrocarbons are desorbed during this step. The adsorber R03itself undergoes a purge step at the lowest pressure of the cycle. It isfed with the gas stream coming from the cocurrent depressurization ofthe adsorbers R04 and R05. The adsorber R03 produces a purge gascomposed of 40 to 85 mol % H₂. The purge gas is compressed to thepressure P′ by the compressor of the CPSA unit so as to form the recyclegas, which is introduced cocurrently into the adsorber R06. This recyclegas may optionally be mixed with a secondary feed external to theprocess and/or introduced into a hydrogen-permeable membrane beforebeing introduced into R03. The permeate from the membrane, rich inhydrogen (with a purity of at least 80 mol %), is used to increase theproductivity of the adsorbent by more effective purging of the adsorberR03; to do this, it is mixed with the gas stream coming from thecocurrent depressurization of the adsorbers R04 and R05. The adsorbersR02 and R01 undergo a succession of balancing recompression steps andrepressurization steps by recycling some of the hydrogen produced untilthe adsorption pressure P at the end of compression of the adsorber R01has been reached. The operation of the CPSA during the other phase timesof the cycle is deduced from the above operation: the adsorber R01, thenthe adsorber R02, and so on as far as the adsorber R07 undergo theadsorption phase during the next phase time.

The process according to the invention may be used for treating a gasmixture coming from a hydrodesulfurization unit. It may be employed atvarious locations in a conventional hydrodesulfurization unit. FIG. 2shows a diagram of a conventional hydrodesulfurization (HDS) unit. Aliquid hydrocarbon feed (1) to be treated, which comprisessulfur-containing molecules, is introduced into the hydrodesulfurizationreactor (HDS reactor) as a mixture with a stream of gas (7) comprisingessentially hydrogen. In the presence of a solid catalyst, manyhydrogenation reactions take place in the reactor and in particularconvert the organic sulfur-containing molecules into hydrogen sulfideH₂S. Leaving the reactor is a two-phase stream (2) that is sent into ahigh-pressure separation unit producing two streams, namely a stream (3)comprising essentially hydrogen and H₂S and a stream (21) comprisingmainly hydrocarbons and H₂S. The stream (21) is then generally expandedand treated in a low-pressure separation unit that produces two streams,namely a stream (22) comprising the hydrocarbons produced by the HDSunit and a stream (35) comprising essentially H₂S. The latter stream(35) is injected into the fuel/sour gas line of the refinery. The stream(3) may be divided into two portions:

-   -   one portion (5) is compressed by the recycling compressor (C2)        so that it can be reused in the HDS reactor—this is the        recycling gas (6),    -   the other portion (33) is sent into one of the sour gas lines of        the refinery. The line (33) includes a purge valve, which may be        opened, for example to regulate the pressure of the unit.

The HDS unit is also fed with a fresh hydrogen stream (11) (this is themake-up gas) which is tapped off the hydrogen line of the refinery andthen, if necessary, compressed by the make-up compressor (C1) up to thepressure of the HDS reactor in order to generate the stream (12). Thecompressed recycling gas (6) and the compressed make-up gas (12) formthe hydrogen-rich gas stream (7) introduced into the HDS reactor.

In a first variant of the invention, the process according to theinvention may be used for treating the high-pressure hydrogen/H₂S gasmixture (3) coming from the high-pressure separation unit of thehydrodesulfurization unit. This variant is illustrated in FIG. 3. Thehigh-pressure hydrogen/H₂S gas mixture (3) is introduced into the CPSAunit and treated according to the process for the invention. During theadsorption phase of the PSA unit, a hydrogen-rich gas (4) is recovered,at the outlet of the adsorber bed, with a pressure loss of less than 1bar, which gas can be sent to the recycling compressor (C2) and then tothe HDS reactor. During the decompression phase of the PSA unit, a wastegas (32), concentrating H₂S and hydrocarbons, is recovered and may befed into one of the fuel/sour gas lines of the refinery.

According to this first variant, it is possible to treat only a portionof the high-pressure hydrogen/H₂S gas mixture (3) thanks to the bypass(41) of the CPSA unit. The line (33) may be opened in order to purge afraction of the gas produced by the process according to the invention.According to this first variant, the process may be implemented in twooptional modes.

According to a first optional mode, all or part of the effluent from thelow-pressure separation section (31) may be mixed with the purge gascoming from the CPSA purge phase before it is introduced into the CPSAcompressor and then optionally into the CPSA membrane. The low-pressuregas (31) may also come from another source having a hydrogen content ofat least 30 vol %. This makes it possible to further reduce the hydrogenlosses of the HDS unit. According to a second optional mode, all or partof the hydrogen-rich gas coming from the CPSA unit (4 then 36) may besent to the existing multistage make-up compressor (C1) (for examplebetween two compression stages). This makes it possible to relieve therecycling compressor (C2), which may sometimes be limited (for exampleowing to the low molecular weight of the gas to be compressed because ofthe purification).

In a second variant of the invention, the process may be used fortreating the high-pressure hydrogen/H₂S gas mixture (5) coming from thehigh-pressure separation section when this mixture is at the intake ofthe recycling compressor (C2). This variant is illustrated in FIG. 4. Inthis case, the CPSA unit treats the high-pressure hydrogen/H₂S gasmixture (5) at the intake of the recycling compressor (C2) and purifiesit in order to produce:

-   -   a hydrogen-rich gas (4), with a pressure loss of less than 1        bar, sent to the recycling compressor (C2) and then to the HDS        reactor; and    -   a waste gas (32) concentrating H₂S and the hydrocarbons and        feeding one of the fuel/sour gas lines of the refinery.

It is possible to treat only a portion of the high-pressure hydrogen/H₂Sgas mixture (5) thanks to the bypass (41) of the CPSA unit. The purgevalve (33) may be opened (for example to regulate the pressure of theunit) and then purges a fraction of the feed gas of the CPSA unit. Twostreams are shown in dotted lines, corresponding to two optional modes.In a first optional mode, the CPSA unit may treat, in addition to thehigh-pressure hydrogen/H₂S gas mixture (5), all or part of the effluentcomprising essentially H₂S from the low-pressure separation section (35then 31), which further reduces the hydrogen losses of the HDS unit.This stream is sent to the intake of the CPSA compressor and thenthrough the CPSA membrane. In a second optional mode, all or part of thehydrogen-rich gas (4) coming from a CPSA unit may be sent to theexisting multistage compressor (C1) (for example between two compressionstages) so as to relieve the make-up compressor (C2).

In a third variant of the invention, the process may be used fortreating the compressed recycling gas (6) delivered by the recyclingcompressor (C2). It purifies it in order to produce:

-   -   on the one hand, a hydrogen-rich gas (8), with a pressure loss        of less than 1 bar, which is sent to the HDS reactor; and    -   on the other hand, a waste gas (32) rich in H₂S and in        hydrocarbons, which is sent into one of the fuel/sour gas lines        of the refinery. This variant is illustrated in FIG. 5. It is        possible to treat only a portion of the compressed recycling gas        (6) owing to the presence of a bypass (41) of the CPSA unit. The        purge valve (33) may be opened (for example to regulate the        pressure of the unit) and may purge a fraction of the gas coming        from the high-pressure separation section. In one optional mode,        all or part of the effluent comprising essentially H₂S from the        low-pressure separation section (35 then 31) may be mixed with        the gas for purging the adsorbent bed before it is introduced        into the compressor and then optionally into the membrane. This        makes it possible to further reduce the hydrogen losses of the        HDS unit.

In a fourth variant, the process may be used to treat the gas mixture(7) consisting of the recycling gas (6) and of the make-up gas (12),(coming from the hydrogen line). This variant is illustrated in FIG. 6.The process produces a gas (8) comprising essentially hydrogen, with apressure loss of less than 1 bar, which is sent into the HDS reactor,and a waste gas (32) concentrating H₂S and the hydrocarbons and feedingone of the fuel/sour gas lines of the refinery. This configuration isuseful when the purity of the make-up gas is low. It is possible totreat only a portion of the mixture (7) consisting of the recycling gas(6) and of the make-up gas (12) owing to the presence of a bypass (41).The purge valve (33) may be opened (for example to regulate the pressureof the unit) and may purge a fraction of the gas coming from thehigh-pressure separation section. In an optional mode, all or some ofthe effluent comprising essentially H₂S from the low-pressure separationsection (35 then 31) may be mixed with the gas for purging the adsorbentbed before it is introduced into the compressor and then optionally intothe membrane. This makes it possible to further reduce the hydrogenlosses of the HDS unit.

The process for the invention makes it possible to achieve hydrogenyields in the recycling gas of greater than 95%. The hydrogen-selectivemembrane makes it possible to reduce the energy needed to compress thepurge effluent and to increase the productivity of the adsorbent, or toreduce its size.

The invention therefore allows purification of the recycling gas of ahydrodesulfurization unit, combining at the same time high purity (atleast equal to 97 mol %), a high yield (greater than 95%) and theproduction of hydrogen under pressure (the pressure difference betweenthe gas mixture to be treated and the purified hydrogen obtained beingless than 1 bar). The process according to the invention thereforeallows the complete treatment of the recycling gas and optionally of themake-up gas. The induced impact on the refining unit is thus very muchgreater than that obtained with any other solution of the prior art. Theinvention makes it possible to obtain in the hydrodesulfurization unit ahydrogen partial pressure between +10% and +60% and an H₂S partialpressure at the outlet of the reactor between −50% and −80%. This impacton the partial pressures is manifested by an improvement in theperformance, which can be expressed in various ways:

-   -   the sulfur content in the hydrocarbons produced by the        hydrodesulfurization unit is reduced by −30 to −80%;    -   the operating temperature is equivalent to the base temperature        of +10 to 25° C.; and    -   the amount of catalyst consumed is reduced by 35 to 50%.

EXAMPLE

The table below compares the results obtained during the use of aconventional PSA unit, by implementing the process according to theinvention without a hydrogen-selective membrane and with ahydrogen-selective membrane, the volume of adsorbent being the same. Thegas mixture treated was composed of 90 mol % H₂, 2 mol % H₂S and 8 mol %hydrocarbons (including 4 mol % CH₄).

The adsorbent was a combination of at least two adsorbents taken fromthe following families: active carbon, activated aluminas, silica gels,zeolites.

The pressure of the feed gas was 35 bar. The pressure of the waste gaswas 6 bar. The mean temperature of the adsorbent beds was 40° C.

Production Purity of the capacity H₂ produced H₂ yield in Sm³/h PSA98.5% 84.3% 30 000 Process according 98.5% 97.8% 54 000 to the inventionwithout hydrogen- selective membrane Process according 98.5% 98.1% 73500 to the invention with hydrogen- selective membrane

It will be understood that many additional changes in the details,materials, steps and arrangement of parts, which have been hereindescribed in order to explain the nature of the invention, may be madeby those skilled in the art within the principle and scope of theinvention as expressed in the appended claims. Thus, the presentinvention is not intended to be limited to the specific embodiments inthe examples given above.

1. A method which may be used for treating a gas mixture at an initialpressure, wherein said gas mixture comprises H₂S and H₂, comprisingtreating said gas mixture with a treatment device, wherein said devicecomprises a pressure swing adsorption (PSA) unit with an integratedcompressor and wherein each adsorber of said unit comprises a pressureswing cycle, wherein said cycle comprises: a) an adsorption phase,wherein said adsorption phase comprises: 1) contacting said gas mixturewith the adsorber bed so as to adsorb the different components of saidgas mixture; and 2) producing at the top of said bed, a gas consistingessentially of hydrogen; b) a decompression phase, wherein saiddecompression phase comprises: 1) introducing a recycle gas to said bed,wherein said recycle gas is compressed by said compressor to a secondpressure which is less than said initial pressure; and 2) producing aPSA waste gas; c) a purge phase, wherein said purge phase comprisesproducing a purge gas; and d) a repressurization phase.
 2. The method ofclaim 1, wherein said recycle gas comprises at least one member selectedfrom the group consisting of: a) said purge gas; and b) said PSA wastegas.
 3. The method of claim 1, wherein said gas mixture comprises astream from a hydrodesulfurization process.
 4. The method of claim 1,wherein said gas consisting essentially of hydrogen has a pressure about1 bar less than said initial pressure.
 5. The method of claim 1, whereinsaid gas comprises a purity greater than about 97% mol.
 6. The method ofclaim 3, further comprising recycling said gas into saidhydrodesulfurization process.
 7. The method of claim 1, wherein saiddecompression phase further comprises decompressing said bed.
 8. Themethod of claim 7, wherein said decompression phase further comprises:a) producing a second gas stream consisting essentially of hydrogen; andb) using said second gas stream, wherein said using comprises at leastone member selected from the group consisting of: 1) using during saidpurge phase; and 2) using to recompress a downstream adsorbent bedduring said repressurization phase.
 9. The method of claim 8, whereinsaid production of said second gas stream corresponds to saidintroduction of said recycle gas.
 10. The method of claim 1, furthercomprising treating a membrane gas in a hydrogen-permeable membraneprior to using said membrane gas as said recycle gas, wherein saidmembrane gas further comprises at least one member selected from thegroup consisting of: a) said purge gas; and b) said waste gas.
 11. Themethod of claim 1, wherein said gas mixture comprises a high-pressuregas mixture from a high-pressure separation unit of ahydrodesulfurization unit.
 12. The method of claim 11, wherein saidhigh-pressure gas mixture is from the intake of the recycling compressorof said hydrodesulfurization unit.
 13. The method of claim 3, whereinsaid gas mixture comprises a compressed recycling gas delivered by therecycling compressor of a hydrodesulfurization unit.
 14. The method ofclaim 3, wherein said gas mixture comprises a mixture of said recyclinggas and the make-up gas of a hydrodesulfurization unit.
 15. A method forthe hydrogenation and treatment of a hydrocarbon feed, comprising: a)creating products by hydrogenating said hydrocarbon feed; b) obtaining agas phase consisting of essentially H₂, H₂S and light hydrocarbons, bycondensing said products; c) treating said gas phase by with a treatmentdevice, wherein said device comprises a pressure swing adsorption (PSA)unit with an integrated compressor and wherein each adsorber of saidunit comprises a pressure swing cycle, wherein said cycle comprises: 1)an adsorption phase, wherein said adsorption phase comprises: i)contacting said gas phase with the adsorber bed so as to adsorb thedifferent components of said gas phase; and ii) producing at the top ofsaid bed, a gas consisting essentially of hydrogen; 2) a decompressionphase, wherein said decompression phase comprises: i) introducing arecycle gas to said bed, wherein said recycle gas is compressed by saidcompressor to a second pressure which is less than said initial pressureof said gas phase; and ii) producing a PSA waste gas; 3) a purge phase,wherein said purge phase comprises producing a purge gas; and 4) arepressurization phase; and d) recycling said gas produced at said topof bed into said hydrocarbon feed.